It is about integrity.
This is a quote from the Shido Yoron where integrity is compared with the joint in a bamboo stem :
"The setsu of the samurai (shisetsu).
When your shiki (samurai ki) is overflowing you should go on to work on your setsu. Setsu is a name which is taken originally from the joints of bamboo; although bamboo has the spirit to grow and pierce the heavens, if it was not for the tightness of the joints it would not be able to endure the frost and the snow, go through all the four seasons and change its colours. If the spirit of the samurai lacks setsu how can he function properly?"